1. Field of the Invention
The present invention relates to a channel selector apparatus suited to be used for an electronic tuner having a search function in a television receiver.
2. Description of the Prior Art
For the channel selector of television receivers, there have been hitherto known and widely used search type channel selectors and preset search type channel selection apparatus. In the former case, tuning voltage applied to the electronic tuner is swept or scanned to seach a desired television or TV signal. Upon reception of the TV signal of a desired channel, the sweeping operation is stopped. On the other hand, in the case of the preset search type channel selector, a ditigal signal is converted into an analog signal for sweeping the tuning voltage applied to the tuner. When the desired signal has been found, optimum tuning point for the reception of the signal is detected, whereupon the supply of the digital signal is interrupted. The channel selection apparatus of the prior art are, however, disadvantageous in that erroneous operations will be often involved particularly when the signal in concern is of a feeble intensity or when unfavorable conditions exist for the signal reception.
More specifically, it is common in the channel selector apparatus that the identity of a desired signal is discriminated by making use of variation in an automatic frequency control or AFC voltage applied to the tuner. For example, referring to FIG. 1 which graphically illustrates variation in AFC voltage in a form of so-called S-like curve, the tuning frequency is taken along the abscissa, while the AFC voltage is taken along the ordinate. As the tuning frequency is swept from a low to high frequency with the tuning voltage being correspondingly swept, an abrupt variation or change in the AFC voltage occurs upon reception of signal. That is, the AFC voltage which is at a high level in the quiescent state is lowered remarkably in response to the signal reception. Subsequent increase in the tuning frequency will result in increasing again the AFC voltage. In the hitherto known channel selectors described above, the sweeping direction of the tuning frequency is inverted when the AFC voltage has attained a predetermined voltage E.sub.o at a frequency f.sub.2 as indicated by a point P after having passed through a point Q at a frequency of f.sub.1 , whereby the sweeping of the tuning frequency is made toward a low frequency. Consequently, the AFC voltage goes once below the voltage level E.sub.o and subsequently rises up again. When the AFC voltage has attained in the course of the reversed sweeping the predetermined voltage level E.sub.o at the frequency f.sub.1 as indicated by the point Q, the sweeping of the tuning voltage is then stopped. In other words, the frequency f.sub.1 at the point Q in the graph of FIG. 1 is selected as the optimum tuning frequency, the reason for which can be explained by the fact that the tuner is capable of receiving the signal in the most stabilized state at the frequency f.sub.1 at the point Q. The frequency sweep is carried out slowly after the inversion of the sweeping direction.
Assuming that the behavior of the AFC voltage is digitalized and the level of AFC voltage higher than the reference E.sub.o is represented by logic "1", while the AFC level lower than E.sub.o is represented by logic "0", the corresponding changes in the logic value during the sweeping of the tuning frequency will be such as shown in FIG. 2(a), in which time is scaled along the abscissa. As can be seen from FIG. 2(a), the logic value of the AFC voltage will change over from "1" to "0" at a time point t.sub.1 at which the tuning frequency has attained the frequency f.sub.1. Subsequentyly, at time t.sub.2 corresponding to the frequency f.sub.2, the logic level of the AFC voltage will restore to "1". In the course of the inverted sweeping, when the tuning frequency is decreased below the frequency f.sub.2 at time t.sub.3, the logic level of the AFC voltage will be reset to "0". When the tuning frequency is further decreased until the frequency f.sub.1 has been attained again at time t.sub.4, the logic level "1" of AFC voltage will be then re-established. At this time, the sweeping operation is stopped.
In conjunction with the above description, it should be noted that the AFC circuit is in general a high impedance circuit and that noise is frequently superposed on the AFC voltage. Accordingly, there will arise such situation that the AFC voltage which has taken the logic "1" level at the time point t.sub.2 will undergo variation in respect of the magnitude and hence the logic value due to noise, as illustrated in FIG. 2(b). Under such circumstance, the AFC voltage could not regain the point Q. Nevertheless, the sweeping of the tuning frequency is interrupted at time t.sub.5, which means that the correct or optimum tuning can not be established. Of course, the noise components superposed on the AFC voltage may be removed by means of a capacitor. However, connection of such filter capacitor will in turn result in an unacceptably increased time constant, to involve slower variation in the AFC voltage as well as a low sweeping rate.